Ongoing research provides further understanding of the biology of RNase A ribonucleases that promote innate immunity with efforts focused toward understanding their mechanisms of action in health and disease. The eosinophil secretory mediators, EDN and ECP, and their mouse orthologs, the eosinophil-associated RNases, feature prominently in our discussion of eosinophils and their roles as immunomodulatory mediators in health and disease in this chapter as a part of this prominent series. Rosenberg, H. F. 2016. Chapter 03007. Eosinophils. In Encyclopedia of Immunobiology, Gordon, J. H. and M. C. Ratcliffe, eds. Amsterdam: Elsevier Publishers.